1. Field of the Invention
This invention relates to a method for automatically identifying a range of digital values representing an x-ray intensity distribution radiograph and more particularly to a method for identifying a range of digital values representing useful diagnostic image information.
2. Background of the Invention
In the field of digital medical imaging, particularly in direct radiography, radiation detectors that capture a portion of the incident radiation and convert it to an electronic charge are often used for image acquisition. The dynamic range of these devices can be made linear over a 10,000:1 range as compared to a 40:1 range for x-ray film. Because of this wide range of detectable exposures, the necessity of re-imaging a patient due to improper selection of exposure factors is virtually eliminated. With the decoupling of the acquisition media from the display device, however, it is necessary to develop a mapping function, often in the form of a look-up table (LUT), which will optimally render the image on the desired output medium (CRT display, film, reflection print).
In developing the optimal gray scale, it has been observed that several key factors contribute to the acceptability of the resulting image. First, it is desirable to present any unattenuated x-ray exposure regions as black background. Second, it is necessary to display different parts of the anatomy at a specific optical density, for example, it may be desirable to be able to perceive the skin-line in various exams, such as for exams of extremities. To be successful at meeting these requirements, one needs to identify the range of digital values representing all or a portion of the desired image features to display, as well as identify the intensity range of the background pixels. Having done this, an exam specific, gray scale algorithm can automatically make the necessary adjustments to ensure that both criteria are met.
The problem of background detection falls into the domain of medical image segmentation. Two general methodologies exist; 1) histogram analysis, and 2) spatial texture analysis (possible combined with gray level intensity information provided by the histogram). Of the first class of methods, U.S. Pat. No. 5,046,118, inventors Ajewole, et al., discloses a method which uses the concept of partial entropy to divide the histogram into a background region and a non-background region. U.S. Pat. No. 5,164,993, inventors Capozzi et al., refers to the latter method and uses it on both the linear and logarithmic histograms, with some additional provisions for conditions when the background point is found at the top of a peak. U.S. Pat. No. 5,596,654, issued to Tanaka et al. on Jan. 21, 1997, discloses a method for finding background and foreground (which are areas of an image that have received very little radiation due to the use of radiation limiting devices such as collimator blades) using a histogram divided into a number of sections by an automatic thresholding procedure. In Tanaka, a discriminant analysis, combined with information about the exam type, exposure technique and desired body portion to be displayed, is then used to adjust the separation points between the sections until the desired ranges for the foreground, object, and background regions are found.
As part of the second group methodologies, U.S. Pat. No. 5,268,967, inventors Jang et al., discloses a four step method which involves morphological edge detection, block classification, and block refinement.
The methods disclosed in Ajewole, Capozzi, and Tanaka, above, may not be useful when the background is varying in a nonuniform way or when multiple background peaks exist in the histogram of the image. The method disclosed in Jang is useful in certain applications but complex processing stages are involved which are time consuming. The texture analysis method is very slow when running in a software implementation and only modestly reliable. There is still need for a practical way to automatically consistently reproduce the correct gray scale of radiographs captured by radiation detectors which produce an electrical signal in response to radiation exposure, particularly in the case where background scatter as in the case of collimated exposures tends to produce undesirable images.
Therefore, it is an object of this invention to provide, prior to exam-specific processing, a method of automatically identifying the range of useful digital values representing the anatomical region of interest to be used for diagnostic display.